The present invention relates to a girder for a bridge or for a construction use and a method of adjusting the load bearing capacity of a bridge using the same, and more particularly, to a prestressed concrete girder having an adjustable load bearing capacity so that the tension of a steel wire is adjusted as necessary, for example, the tension of a steel wire is decreased as the load increases during construction, or there is a need to compensate for sagging or cracking of the girder due to a long-term load after construction, and to a method of adjusting the load bearing capacity of the bridge using the above girder.
A prestressed concrete (PSC) girder for a bridge has been used over 40 years and is widely used for a bridge having a span of 50 m and less in many countries. Recently, the length of the girder gradually increases as the width of roads increases. Girders of 40 m and more up to 95 m have been recently developed and used in the U.S. and the use thereof gradually increases. Such girders having a long span often use high strength concrete or a bulb T-shape profile having a large sectional coefficient. With an increase of use of the long-span girder, the U.S. Federal Road Administration suggested six series of the same kind of a profile to be used for a span of 20-30 m. Also, in 1988, the Administration suggested three standard profiles for a long-span girder in cooperation with the U.S. Prestressed Concrete Academy. Thereafter, various profiles of applications of the above standards are developed and used by the respective state of the U.S. and many universities. Accordingly, although the number of bridges constructed in the U.S. generally decreases, portion of structures including bridges using the prestressed concrete girder has been gradually increased.
Also, al the girders installed above piers of a bridge wear out for a long time or weighty vehicles exceeding a designed weight pass over the bridge, mold is damaged and an excess of sagging is generated. Here, a bending tension crack is generated together. When the damage continues, since the bridge finally collapses, appropriate repair and/or reinforcement of the bridge is required.
The repair and reinforcement of the PSC bridge is performed in an external steel wire reinforcement method in which steel wires installed outside must be fixed in an appropriate method. However, since it is difficult to install a fixing apparatus at an end portion of the girder and the reliability of the load bearing capacity of the fixing apparatus is not guaranteed, other various methods are used but no perfect apparatus has not been developed yet. Thus, when a crack an/or sagging is generated in the PSC bridge, the repair and reinforcement thereof is difficult. It will be a very advantageous merit that a girder already includes an apparatus to easily adjust or increases a level of the load bearing capacity of a bridge as necessary to overcome the above problems.
Also, the weights of vehicles gradually increase with an increase of the traffic amount and the development of vehicle manufacturing technologies or overall industries. When the weights of vehicles increase, the specification which is a standard of design must be modified accordingly. The design standard is established or revised by the Ministry of Construction and Traffic and there was a very significant revision of the specification in 1982. In the revision, the grade of a bridge is classified into three levels and the designed weight of the 1st level is adjusted to 43 tons from 32 tons while the designed weight of a second level bridge is 32 tons. Such revision of the specification necessarily entails a state of unbalanced load bearing capacity in which that of the existing bridges do not match one another. That is, roads on which 43 ton trucks can ride and roads on which the 43 ton trucks cannot ride are mingled so that the efficiency of a nationwide transportation network is severely damaged. Thus, it is requested to seek an economic reinforcement method to increase the load bearing capacity of the second level bridges, occupying over 50% of the nation, to the first level bridges to coordinate the load bearing capacity of these bridges.
The width of roads is generally increased as the number of lanes in a road increases. Accordingly, the development of a long-span bridge for construction of elevated roads or overpasses crossing such wide roads is currently performed. Also, a prepollex beam has been domestically developed, but manufacturing and carriage thereof is difficult because it is too long and the price thereof is very expensive compared to the existing PSC beams.
Also, it is a recent trend to use high strength concrete because the long-span girder is manufactured. Accordingly, due to the application of high tension, the amount of creeps generated is very large. As the creep increases, the girder sags more, which affects the vertical alignment of the overhead road. When the vertical alignment is deteriorated, an impact coefficient due to vehicles passing the road additionally increases. Thus, in the case of the high strength girder or long-span girder, after a long-term use thereof, a repair to compensate for sagging through an appropriate method will be needed.
FIG. 1 is a view showing the structure of a bridge according to a conventional technology.
As shown in FIG. 1, according to the conventional technology, a plurality of I-type girders 12 are installed over a pier 10 and an upper plate slave (not shown) is installed above the girder 12.
FIG. 2 is a sectional view showing the arrangement of steel wires in the girder according to the conventional technology.
As shown in the drawing, the section of the girder 20 which is an I-type girder is formed of a body portion 22, an upper flange 28 and a lower flange 24. Also, a tensioning member 26 which is a plurality of steel wires is installed in the lower portion of the body portion 22 and the lower flange lengthwise with respect to the girder 20. An upper plate of a bridge is installed above the upper flange 28 and the bottom surface of the lower flange 24 is supported by the pier 10.
In the case of the I-type girder 20 according to the conventional technology, after construction is completed, when sagging or crack is generated due to passing of vehicles, thus damaging the bridge, so that the bridge needs to be repaired or the designed passage load needs to be increased according to a revision of the specification, it is a problem that there is no appropriate economic way to reinforce the girder.
To solve the above problems, it is one object of the present invention that a prestressed concrete girder having an adjustable load bearing capacity by which when an excess of sagging or a crack is generated to a is bridge due to long-term deterioration or overload, the sagging of a girder and crack can be compensated for by releasing the tension of the steel wires provided to the upper flange step by step, or when there is a need to increase the load bearing capacity of the bridge without any particular damage to the bridge, the load bearing capacity of the bridge can be easily increased with no special equipments, and to a method of adjusting the load bearing capacity of a bridge using the same.
Also, it is another object of the present invention that a prestressed concrete girder having an adjustable load bearing capacity by which, during construction, the steel wires are released step by step according to an increase of load to reduce the height of a mold of the girder or increase the span, and to a method of adjusting the load bearing capacity of a bridge using the same.
To achieve the above objects, there is provided a prestressed concrete girder having an adjustable load bearing capacity in a bridge comprises at least one non-attached steel wire installed at an upper flange of the girder in a lengthwise direction of the girder, in which the heigh of the bridge is reduced, the span of the bridge is increased, or a long-term crack or sagging of the bridge is compensated for by adjusting tension of the steel wires when the bridge is under construction, or after laying of slab or completion of construction.
It is preferred in the present invention that the upper flange comprises a cut-away portion formed at a predetermined portion thereof and through which the steel wires pass, in which the cut-away portion is always exposed so that the steel wires can be cut as necessary after construction is completed, and in which the number of steel cores forming the steel wires exposed to the outside at the cut-away portion by cutting or releasing some of steel cores so that the tension of the steel wires can be adjusted.
Thus, according to the present technology, the tension of the girder can be adjusted so that the above problems can be solved.